Photographic elements typically comprise a plurality of photosensitive layers and non-photosensitive layers, such as, interlayers, anti-halation layers, protective layers and the like. The layers generally are coated from fluid solutions containing a hydrophilic colloid, such as gelatin. Usually, coating takes place at temperatures between about 35.degree. to about 50.degree. C. The fluid solutions are frequently referred to as "melts" or "coating melts".
Generally, in the photographic art there are two primary methods of coating photographic compositions. One is the bead coating process. U.S. Pat. No. 2,761,417 to Russell et al., U.S. Pat. No. 2,681,294 to Beguin, and U.S. Pat. No. 4,525,392 to Ishizaki illustrate simultaneously applying multiple layers of photographic compositions by bead coating processes, and apparatus for practicing those processes.
The second primary method is the curtain coating process. U.S. Pat. No. 3,632,374 to Greiller, and U.S. Pat. No. 4,569,863 to Koepke et al., illustrate apparatus and processes for curtain coating.
It is well known that adjustment and control of viscosities of typical photographic coating melts of individual layers can improve uniformity of layer thickness of the coated product. It is also known that layer viscosities outside optimum ranges may cause undesired variations in layer thicknesses during flow on the slides of the coating hopper or on a non-horizontal web path after coating.
Coating melts with low gelatin concentrations, below about 4% gelatin, have relatively low viscosities and as a result are particularly difficult to coat in multilayer slide hopper coating machines without undesired variation in layer thickness. Usually, in multilayer curtain coating devices even higher viscosities are desirable for producing uniform coatings.
There are certain advantages, however, in using low gelatin-containing melts. In general, the lower the gelatin content, the thinner the resulting coating. Thinner layers in photographic coatings provide the advantages of (1) sharper images, and (2) faster processing films.
It has been proposed to incorporate polymeric materials into gelatin-containing photographic coating compositions to increase the viscosity thereof. See for example U.S. Pat. Nos. 3,861,924 to Mackey et at and 3,926,869 to Horie et al and Soviet patent publication SU 857,917 to Korneva. However, the use of polymeric materials can add to the cost of the product as polymers with sufficient purity and specific molecular weight or molecular weight distribution for photographic use are expensive to manufacture.
Therefore, there is a need to control the viscosity of gelatin-containing photographic melts in a cost effective manner and in particular to formulate high viscosity, low gelatin-containing photographic melts that are suitable for producing relatively thin, uniform multilayer product coatings, especially in multiple layer slide hopper or curtain coating machines.